This invention relates to methods of extruding plastic material in multi-layer tubular form.
Multi-layer plastic films are becoming in more demand for a variety of commercial uses. The various layers of a multilayer plastic film are selected to provide the film with required properties, for example with respect to strength, moisture impermeability and oxygen impermeability. In view of the increasing commercial demand for such films, improved methods for their production are required.
It s therefore an object of the invention to provide an improved method for producing multilayer plastic film.
According to the one aspect of the invention, a method of extruding plastic material in multi-layer tubular film includes providing an extrusion die having a first body member and a second body member surrounding the first body member and forming an annul passage therebetween which communicates with an annular extrusion orifice, said body members having a series of helical grooves extending around the die and each commencing at a separate location, said separate locations being spaced around a lower portion of the first body member, a single encapsulating unit for providing a flow of encapsulated plastic material comprising a first plastic material surrounded by a second plastic material at an outlet thereof, said first body member having a series of cross-holes extending from said encapsulating unit output and each communicating with a respective separate location, and feeding encapsulated plastic material from the outlet of the single encapsulating unit through the cross-holes to the helical grooves and subsequently to the annular passage for extrusion through the annular extrusion orifice as a multi-layer film in which the layers are inclined to and extend to the opposite surfaces of said film.
Preferably, each helical groove extends around the inner body member of the extrusion die for at least two cross-holes and advantageously for at least 360xc2x0.
A third plastic material may be supplied to surround the encapsulated plastic material and produce a multi-layer plastic film in which the layers are inclined to and extend to the opposite surfaces of said film.
A further plastic material may be supplied to the inside and/or outside of the multi-layer plastic film before extrusion through the annular extrusion orifice to produce a multi-layer plastic film in which the layers are inclined to and extend to the outer surfaces thereof and at least one of said outer surfaces is covered by a layer of said further plastic material.
According to another aspect of the invention, an annular extrusion die for extruding plastic material in multi-layer tubular form has a first body member and a second body member surrounding the first body member and forming an annular passage therebetween which communicates with an annular extrusion orifice, said body members having a series of helical grooves extending around the die and each commencing at a separate location, said separate locations being spaced around a lower portion of the body member, and a single encapsulating unit for providing a flow of encapsulated plastic material comprising a first plastic material surrounded by a second plastic material at an outlet thereof, said first body member having a series of cross holes extending from said encapsulating unit output and each communicating with a respective separate location whereby the encapsulating unit supplies encapsulated plastic film through the outlet and the cross holes to the helical grooves and subsequently to the annular passage for extrusion through the annular extrusion orifice as a multi-layer tubular film in which the layers are inclined to and extend to the opposite surfaces of said film.
According to a still further aspect of the invention, a multi-layer plastic film has parallel opposite outer surfaces in which the layers are inclined to and extend to said outer surfaces. The plastic film may also include an outer layer of plastic material covering one or both outer surfaces.